Fluorescent conversion films using a phosphor or fluorescent material which are capable of converting light emitted from a light source into light having a different wavelength have been extensively used in various application fields such as electronic displays. For example, there are disclosed organic electroluminescent devices having an organic electroluminescent material portion emitting blue light or bluish green light, and a phosphor portion absorbing the light emitted from the light-emitting layer and emitting a visible fluorescence having at least one color ranging from bluish green to red (e.g., refer to Japanese Patent Application Laid-open No. 152897/1991). Also, there are disclosed red color fluorescent conversion films obtained by dispersing a rhodium-based fluorescent pigment and a fluorescent pigment having an absorption band in a blue light range and being capable of inducing transfer or reabsorption of energy into the rhodium-based fluorescent pigment, in a light-transmitting medium (e.g., refer to Japanese Patent Application Laid-open No. 286033/1996). In these fluorescent conversion films, there have been used organic fluorescent pigments having a cycloalkyl group and/or a heterocyclic ring as a steric hindrance group, for example, as disclosed in Japanese Patent Application Laid-open No. 44824/2000.
However, these conventional techniques have the following problems.
(1) In order to enhance a color purity of light emitted from devices, light emitted from an excitation light source must be converted into light having the other wavelength by a fluorescent conversion film at a high efficiency. If the light emitted from the excitation light source is transmitted through the film without conversion, the color purity thereof tends to be lowered. For the purposes of enhancing a conversion efficiency of the fluorescent conversion film and increasing an intensity (fluorescence intensity) of the converted light, it is required to allow the film to sufficiently absorb the light emitted from the excitation light source. However, if the concentration of organic fluorescent pigments contained in the fluorescent conversion film is increased for allowing the film to sufficiently absorb the light, undesired association between the organic fluorescent pigments tends to occur in the film, so that energy absorbed from the light source is transferred to the adjacent pigments. As a result, a so-called concentration quenching phenomenon is inevitably caused, thereby failing to attain a high fluorescence quantum yield.
(2) As the light-transmitting medium, reaction-curable resins such as photocurable resins and heat-curable resins have been mainly used in view of achieving improvements in heat resistance or productivity of the fluorescent conversion films. In this case, there tends to arise such a problem that reactive components contained in the resins are reacted with the organic fluorescent pigments, resulting in decomposition of the pigments or change in structure of the films and, therefore, further deterioration in fluorescent properties thereof.
The above problem is more specifically explained below on the basis of experiments. In FIG. 16, there is shown the relationship between an absorbance and a fluorescence quantum yield of the fluorescent conversion film when varying a concentration of pigments contained in the film wherein the circle mark (◯) indicates the fluorescent conversion film obtained by dispersing rhodamine 6G as an organic fluorescent pigment in a benzoguanamine resin; the triangle mark (Δ) indicates the fluorescent conversion film obtained by dispersing coumarin 6 as an organic fluorescent pigment in a benzoguanamine resin; and the solid triangle mark (▴) indicates the fluorescent conversion film obtained by dispersing coumarin 6 as an organic fluorescent pigment in a photocurable resin. Meanwhile, the rhodamine 6G-dispersed film was irradiated with light from a light source having a peak at 534 nm, whereas the coumarin 6-dispersed films were irradiated with light from a light source having a peak at 456 nm. In FIG. 16, the abscissa axis represents an absorbance at the wavelength, whereas the ordinate axis represents a fluorescence quantum yield.
As apparently shown in FIG. 16, when the concentration of the pigments in the fluorescent conversion film is low, the fluorescence quantum yield of the film is as low as less than 50% in the range where the absorbance to the excited light is more than 1 even when using such pigments having a fluorescence quantum yield as high as 80% or more. In particular, in the case where the pigments are dispersed in a photocurable resin as a reactive resin, it is recognized that the fluorescence quantum yield of the film is as low as about 30%.
To solve the above problems encountered upon using the organic fluorescent pigments, there are disclosed techniques using inorganic phosphors in place of the organic fluorescent pigments (e.g., refer to Japanese Patent Application Laid-open Nos. 199781/1999 and 181419/1999 and U.S. Pat. No. 6,608,439).
In Japanese Patent Application Laid-open No. 199781/1999, cerium-activated yttrium/aluminum/garnet-based phosphors (so-called YAG:Ce phosphors) are used as the inorganic phosphors, and the phosphors are dispersed in a thermoplastic resin sheet to form a fluorescent conversion film. In the above Japanese Patent Application, since a baked YAG:Ce molded product is pulverized to produce phosphor microfine particles used in the fluorescent conversion film, the resultant microfine particles have a particle size in the order of a micron meter. For this reason, in order to allow the fluorescent conversion film to fully absorb light emitted from an excitation light source without scattering the light, it is required to increase a thickness of the resin sheet, for example, up to a thickness as large as 120 μm, and disperse the microfine particles therein at a low concentration. As a result, it is difficult to apply such a resin sheet to a fluorescent conversion film for organic electroluminescent devices.
In Japanese Patent Application Laid-open No. 181419/1999, there is disclosed the process for producing a metal oxide-based phosphor by reacting at least one compound selected from the group consisting of carbonates, nitrates, hydroxides, sulfates, phosphates, borates, silicates, aluminates, carboxylates, halides and alkoxides of a metal element constituting a matrix and an activator of the phosphor with an oxy-carboxylic acid or a polyamino chelating agent to obtain a metal complex, polymerizing the metal complex with a polyol in a solvent to produce a complex polymer, and baking the complex polymer. However, in this method, since the baking is performed at a temperature as high as 800° C. or more, organic components contained in the reaction product tend to be thermally decomposed, so that the particles undergo secondary agglomeration. As a result, the particle size of the obtained phosphor particles is as large as about 100 nm and, therefore, still unsatisfactory. Further, in the method, for the same reason as described above, there tends to arise such a problem that the obtained phosphor particles have a poor dispersibility in organic solvents or resins.
In the literature “APPLIED PHYSICS LETTERS”, Vol. 80, No. 19, pp. 3608 to 3610 (2002), fine particles obtained by sol-gel method are baked at a temperature as high as 800° C. or more, thereby producing a YAG:Ce phosphor having a particle size of about 35 nm. However, similarly to the process described in Japanese Patent Application Laid-open No. 181419/1999, since the obtained particles contains no organic components, there tends to arise such a problem that the obtained particles have a poor dispersibility in organic solvents or resins.
In U.S. Pat. No. 6,608,439, a film obtained by dispersing a nano-crystal of a Group II to VI semiconductor such as cadmium selenide (CdSe) in a resin is used as a fluorescent, conversion film. However, in general, the metal chalcogenide tends to be deteriorated in durability such as water resistance, chemical resistance and heat resistance.